The present invention relates to a plasma processing apparatus capable of being used for dry etching, sputtering, plasma CVD and the like in a process of manufacturing a semiconductor and a thin film circuit, and in particular to a high frequency inductive coupled plasma processing apparatus.
In recent years, to effect processing or the like on a semiconductor element at a high aspect ratio by a dry etching technique or effect burying or the like at a high aspect ratio by a plasma CVD technique, coping with developing dimensional fineness of semiconductor elements, it has been required to effect plasma processing in higher vacuum.
For instance, in the case of dry etching, when a high density plasma is generated in high vacuum, there is a reduced possibility of collision between ions and neutral radical particles in anion sheath formed on a substrate surface, and therefore directions of the ions are aligned toward the substrate surface. Furthermore, because of a high degree of electrolytic dissociation, there results a high incident particle flux ratio of ions to neutral radicals arriving at the substrate. For the above-mentioned reasons, etching anisotropy is improved by generating a high density plasma in high vacuum, thereby allowing processing to be achieved at a high aspect ratio.
Furthermore, in the case of plasma CVD, when a high density plasma is generated in high vacuum, an effect of burying and flattening a fine pattern can be obtained by the sputtering effect with ions, thereby allowing burying to be achieved at a high aspect ratio.
A conventional generic parallel flat plate type plasma processing apparatus will be described with reference to FIG. 7. In FIG. 7, a substrate electrode 4 on which a substrate 5 is to be disposed and an opposite electrode 30 are provided inside a vacuum vessel 3, and a high frequency voltage is applied across the electrodes 4 and 30 from an electrode-use high frequency power source 6, so that plasma is generated inside the vacuum vessel 3. It is to be noted that an electrode-use matching circuit 7 is a circuit for matching a load impedance with a characteristic impedance of an electrode-use connection cable 8.
According to the above-mentioned system, the probability of collisions between electrons and ions reduces according as the degree of vacuum increases. Therefore, it is difficult to generate a high density plasma in high vacuum, and consequently sufficient velocity of processing cannot be obtained. Furthermore, when the high frequency voltage is irrationally increased so as to increase the plasma density, an ion energy increases and this results to possibly reduce etch selectivity or cause damage to the substrate.
In contrast to the above-mentioned parallel flat plate type plasma processing apparatus, there is a high frequency inductive coupled plasma processing apparatus which generates plasma in its vacuum vessel by applying a high frequency voltage to its discharge coil, as a plasma processing apparatus capable of generating a high density plasma in high vacuum. The plasma processing apparatus of the above-mentioned type generates a high frequency magnetic field in the vacuum vessel and generates an induction electric field inside the vacuum vessel by means of the high frequency magnetic field so as to accelerate electrons and generate plasma. By increasing a coil current, a high density plasma can be generated even in high vacuum, and consequently a sufficient velocity of processing can be obtained.
As the high frequency inductive coupled plasma processing apparatus, principally a flat plate type as shown in FIG. 8 and a cylinder type as shown in FIG. 9 are known. In FIGS. 8 and 9, there are shown a flat plate type discharge coil 31, a cylinder type discharge coil 32, a discharge coil-use high frequency power source 9, a discharge coil-use matching circuit 10, and a discharge coil-use connection cable 11. The discharge coil-use matching circuit 10 is connected to each of the discharge coils 31 and 32 by way of a conductor wire 12. It is to be noted that the vacuum vessel 3, substrate electrode 4, substrate 5, electrode-use high frequency power source 6, electrode-use matching circuit 7, and electrode-use connection cable 8 are the same as those of FIG. 7.
In FIGS. 8 and 9, upon applying a high frequency voltage to each of the discharge coils 31 and 32 from the discharge coil-use high frequency power source 9 while introducing an appropriate gas into the vacuum vessel 3 and discharging a gas therefrom with the vacuum vessel 3 kept internally at an appropriate pressure, there is generated plasma inside the vacuum vessel 3, thereby allowing the substrate 5 disposed on the substrate electrode 4 to be processed with plasma processing such as etching, deposition, and surface improvement. In the above place, by additionally applying a high frequency voltage to the substrate electrode 4 from the electrode-use high frequency power source 6, energy of ions reaching the substrate 5 can be controlled.
However, according to the conventional systems shown in FIGS. 8 and 9, there is a great power loss in the discharge coil-use matching circuit 10, and this results in the disadvantages of a lowered power efficiency and a possible temperature rise in the discharge coil-use matching circuit 10.
The above-mentioned disadvantages will be described in detail below. FIG. 10 is a circuit diagram of a representative one of the discharge coil-use matching circuit 10. There are included an input terminal 13, variable capacitors 14 ! and 15, and a matching-use series coil 16. By controlling the capacitances of the variable capacitors 14 and 15 in a feedback manner, the circuit can cope with a very small fluctuation of the load impedance. Depending on the magnitude of the load impedance, it is required to change the number of turns of the matching-use series coil 16, remove the matching-use series coil 16, or insert a fixed capacitor 17 or 18. A reference numeral 19 denotes an output terminal.
FIG. 11 is a Smith chart, in which the hatched area shows a range of matching of the discharge coil-use matching circuit 10 shown in FIG. 10. As might be expected, the range of matching shown in FIG. 11 varies depending on constants of elements in the discharge coil-use matching circuit 10, however, there is shown a representative case as an example. In regard to a complex representation of impedance of the discharge coil 31 or 32, there is shown by a curve A the impedance of the discharge coil 31 or 32 in a case where its imaginary number component is five times as great as a characteristic impedance of the discharge coil-use connection cable 11. It can be found in FIG. 11 that a greater part of the curve A is out of the range of matching.
In view of the above, there is a discharge coil-use matching circuit 10 as shown in FIG. 12 in which is connected a matching-use parallel coil 20 having an impedance equal to that of the imaginary number component of the complex representation of the impedance of the discharge coil 31 or 32. In the above case, an impedance at the load side evaluated from a load-side terminal of the variable capacitor 25 has its imaginary number component being half of the curve A, i.e., two and half times as great as the characteristic impedance of the discharge coil-use connection cable 11. Therefore, the impedance is expressed by a curve B as shown in FIG. 11. Normally, a real number component of the impedance of the discharge coil 31 or 32 is extremely small, and therefore a part of the curve B (most part of a range in which the real number component is not greater than 1.3 times the characteristic impedance of the discharge coil-use connection cable 11) is in the range of matching. Accordingly, it can be understood that matching can be achieved by using the matching-use parallel coil 20.
The above has described the case where the impedance of the matching-use parallel coil 20 is equal to the impedance of the discharge coil 31 or 32. However, when the impedance of the discharge coil 31 or 32 is great, there can be achieved no matching unless the impedance of the matching-use parallel coil 20 is reduced to a considerably small extent. Otherwise, when the impedance of the discharge coil 31 or 32 is not so great, the smaller the imaginary number component of the impedance at the load side evaluated from the load-side terminal of the variable capacitor 15, the wider a margin for the matching results. Therefore, the impedance of the matching-use parallel coil 20 is preferably as small as possible. In such a case, the matching-use parallel coil 20 and the discharge coil 31 or 32 are connected in parallel with each other in terms of circuit construction, and there fore a greater current flows through the matching-use parallel coil 20 than the discharge coil 31 or 32. Therefore, even when the real number component of the impedance of the matching-use parallel coil 20 is a small value, the power loss occurring there cannot be ignored, and the power efficiency is lowered. Since the power loss is equivalent to a calorific value of the matching-use parallel coil 20, a temperature rise in the discharge coil-use matching circuit 10 will result.
Furthermore, in the case of the flat plate type high frequency inductive coupled plasma processing apparatus, there is required a discharge coil 31 having at least the same size as that of the substrate 5 in order to generate plasma inside the vacuum vessel 3 with a good intra-substrate-surface uniformity. In regard to the configuration of the discharge coil 31, there can be considered a one-turn coil as shown in FIG. 13. In general, the inductance of a coil increases according as the diameter of the coil increases, and consequently, the inductance of the discharge coil 31 cannot help increasing when the substrate 5 is large. Furthermore, in order to further improve the intra-substrate-surface uniformity, the configuration of the discharge coil 31 is preferably a spiral coil as shown in FIG. 14. Comparing the spiral coil with the one-turn coil, of course the spiral coil has a greater inductance when the outermost diameters of the discharge coils 31 are approximately identical. According to our measurement, the inductance of a spiral discharge coil 31 such that the uniformity in density of plasma is not greater than 3% at a diameter of 150 mm is 1.1 .mu.H in a certain discharge condition. When the frequency of the discharge coil-use high frequency power source 9 is 13.56 MHz, the imaginary number component of the impedance of the discharge coil 31 is 94 .OMEGA., i.e., slightly smaller than two times a normal value of 50 .OMEGA. of the characteristic impedance of the discharge coil-use connection cable 11. Around the above-mentioned value, it is possible to achieve matching without the matching-use parallel coil 20. However, as described hereinbefore, taking a margin for the matching into account, it is preferable to insert a coil of about 0.5 to 1 .mu.H as the matching-use parallel coil 20 in terms of margin for the matching.
The impedance of the discharge coil 31 is proportional to the frequency. Therefore, for example, when the frequency of the discharge coil-use high frequency power source 9 is 40 MHz, the impedance of the discharge coil 31 having an inductance of 1.1 .mu.H is about 276 .OMEGA. (=50 .OMEGA..times.5.5), meaning that the matching-use parallel coil 20 is indispensable in achieving matching.
In order to generate a uniform plasma for a great area when the substrate 5 is large or when a batch processing is desired robe effected, the discharge coil 31 is of course required to be dimensionally increased. Even when the frequency of the discharge coil-use high frequency power source 9 is 13.56 MHz, the impedance of the discharge coil 31 sometimes becomes several hundred ohms. In this case, the matching-use parallel coil 20 is indispensable for the achievement of matching.
In view of the above, there can be considered a method of connecting a plurality of spiral coils in parallel with each other in a manner as shown in FIG. 15 as a discharge coil arrangement capable of generating a uniform plasma for a great area and reducing the impedance of the discharge coil 31. However, when such a discharge coil arrangement is adopted, high frequency magnetic fields formed by adjacent coils partially cancel each other, consequently causing the disadvantage that a sufficient plasma density cannot be obtained. According to our measurement, when four same spiral coils are used in a parallel connection, a total inductance of the discharge coils is 0.51 .mu.H, i.e., reduced to 59% of that of a case where one spiral coil of 1.3 .mu.H is used, whereas the plasma density is disadvantageously reduced by 11%.
In the case of the cylinder type high frequency inductive coupled plasma processing apparatus, the helical type discharge coil 32 is provided around the vacuum vessel 3, and therefore a helical type discharge coil 32 having at least the same size as the exterior size of the vacuum vessel 3 is to be used. Therefore, generally the inductance of the discharge coil 32 increases as compared with that of the flat plate type high frequency inductive coupled plasma processing apparatus. Therefore, in order to achieve matching or secure a margin for the matching, there are many cases requiring the matching-use parallel coil 20. According to our measurement, when the diameter of the cylinder is 300 mm, the inductance of the helical type discharge coil 32 is 1.8 .mu.H (=150 .OMEGA. at 13.56 MHz).
As apparent from the above description, the matching-use parallel coil 20 is required for the purpose of increasing the size of the processing area, increasing the inductance of the discharge coil 31 or 32 for increasing the application frequency, or increasing the margin for the matching. However, when the matching-use parallel coil 20 is used and particularly the imaginary number component of the impedance thereof is small, there is inevitably produced power loss together with a lowered power efficiency. Since the power loss is equivalent to the calorific value of the matching-use parallel coil 20, a temperature rise in the discharge coil-use matching circuit 10 has resulted disadvantageously.